1. Field of the Invention
The present invention relates to acrylate-methacrylate-graft polymerizates, which serve as thermoformable materials, and the moulded bodies manufactured therefrom; as well as aqueous dispersions of acrylate-methacrylate graft polymerizates and the films and foils manufactured therefrom. The products are characterized by, among other things, very good low temperature strength, good blocking behavior and high cohesion.
2. Description of the Prior Art
Acrylate-methacrylate-graft polymers have achieved wide commercial application, especially as impact modifiers for thermoplastically processable moulding compounds (see, for example, British Patent 975,421).
Such acrylate-based impact modifiers are usually manufactured according to the process of emulsion polymerization, where in the first reaction step a mixture of acrylate and crosslinker is polymerized into a crosslinked polyacrylate-latex particle and in a second process step a hard shell made of polyalkylmethacrylate, usually polymethylmethacrylate (PMMA), is grafted on this crosslinked particle. Usually the hard polymethacrylate shell serves for better handling of the acrylate elastomers and for binding the polyacrylate particles to the plastics to be modified, usually polyvinyl chloride (PVC), since PMMA and PVC are quite compatible and thus, for example, a butyl acrylate rubber can be bonded quite well to PVC by means of PMMA grafted on the butyl acrylate rubber.
By suitably varying the hard polymethacrylate shell of the latex particle, other plastics can also be impact-modified; for example, by grafting a methyl methacrylate-cyclohexyl methacrylate copolymer shell on an acrylate rubber, an entire line of plastics can be impact modified, since this methyl methacrylate-cyclohexyl methacrylate copolymer is compatible with a whole series of plastics (e.g. PVC, polystyrene, PMMA) (see European Published Patent Application 312 878). Especially useful are the crosslinking systems of the acrylic elastomers, where frequently a combination of crosslinkers (butylene glycol diacrylate) and graft crosslinkers (e.g., allyl methacrylate) is used (cf. German Offenlegungsschrift 21 16 653).
Frequently the refractive index of the polyacrylate elastomer phase is also adjusted, through copolymerization with styrene, to the refractive index of the PMMA phase, in order to obtain good optical properties of the impact-modified polymer mixtures (see, for example, European Published Patent Application 113 924).
Special importance is placed, generally, on good crosslinking of the acrylate elastomer, where this crosslinking is frequently characterized by the gel body content or the maximum degree of swelling (Japan Kokai 7624, 689; CA 85: 47521 r (1976)).
In some cases butadiene and other readily graftable monomers are also copolymerized in order to obtain, on the one hand, well crosslinked rubber particles and, on the other hand, a good methacrylate graft.
Similarly, reactive groups to improve the grafting of polymethacrylate and polyacrylate are used. Examples are primarily epoxide groups, for example, glycidyl methacrylate; methylol groups, for example, methylol methacrylamide; maleic acid anhydride; isocyanatoethyl methacrylate and other compounds that can be converted with nucleophilic groups. Also the use of hard radiation to improve the grafting of methacrylates on specified acrylates is described.
Other attempts to obtain well grafted acrylate/methacrylate systems can be seen in studies that try to improve the grafting of methacrylates on the acrylates by means of a multi-step polymerization process with a step-by-step transition from an acrylate to a methacrylate. For example, in French Patent 2,069,007 U.S. application Ser. No. 877,847 a dispersion prepared by:
a first step comprising 99%s C.sub.1 -C.sub.2 alkyl acrylate and 1% butylenediacrylate,
a second step comprising 60% methyl methacrylate (MMA) and 40% (m)ethylacrylate,
a third step comprising 90% methyl methacrylate and 10% (m)ethylacrylate or methoxyethyl acrylate, and
a fourth step comprising a 90:2 mixture of MMA and methylacrylate or methoxyethyl acrylate with 4% methacrylic acid, based on the mixture is described.
Similarly, an acrylate film in which a first step comprising 90% butyl acrylate and 10% MMA and 0.5% triallyl cyanurate as crosslinker-containing mixture is converted with a second step comprising 50% MMA and 50% butyl acrylate and finally a third step comprising 10% butyl acrylate and 90% MMA is described in Japan. Kokai 7864 229 (CA: 89: 1247 576 p). The process is analogous in Japan. Kokai 7733,991 (CA: 87: 543093r).
In European Published Patent Application 56 242 and German Application 31 00 748, an elastomer powder is described that is prepared through graft polymerization from a butyl acrylate latex with methyl methacrylate.
Elastomers, i.e., materials that can be expanded by at least twice their starting length through the effect of a slight force at room temperature and above and following elimination of the force return again rapidly and virtually completely into the original shape, have found a variety of possible applications in technology. Thermoplastically processible elastomers are an especially interesting group.
Usually thermoplastic elastomers represent multiphase systems in which the phases are homogeneously dispersed. The phases are connected multiple times through graft and block polymerization. (Cf. H. F. Mark et al, Encyclopedia of Polymer Science & Technology, Vol. 5, pp. 416-430, J. Wiley 1986). Conceptionally it is assumed that there is at least one hard phase, which can be liquefied while heating, and a soft phase, which behaves like rubber at room temperature. If the impact modifiers have as a rule fine, crosslinked rubber particles in a hard matrix, the thermoplastic elastomers exhibit generally a continuous rubber phase with embedded hard "crosslinking" domains. Whereas the thermoplastic elastomers used primarily in engineering represent predominantly block copolymers with hard systems comprising polystyrene, polysulfone, polyester, polyurethane or polycarbonate and "softer" segments comprising polyolefins, polysiloxane, or polyether, in European Published Patent Application 0 381 065 acrylate-based elastomers are proposed that comprise at least 40 wt. % copolymerizates having a molecular weight &gt;50,000 Daltons, 50-95 wt. % of which are synthesized from .alpha.) acrylate monomers and the rest from .beta.) macromonomers comprising vinylic groups and thus covalently bonded to a polyvinyl unit, selected from the group of acrylates and methacrylates having a glass temperature Tg of at least 60.degree. C. and a molecular weight of 500-100,000 Daltons.
Comb polymers with a polybutyl acrylate main chain and side chains comprising polymethyl methacrylate macromonomers show, for example, the properties of thermoplastically processible elastomers (TPE) that are known from styrene-butadiene-styrene three block copolymers. As interesting as such macromonomer-comb polymers are with respect to their properties, the fact cannot be overlooked that they are very special products of a relatively expensive technology and, presumably, will remain as such. Therefore, a desired objective is to provide thermoplastically processible elastomers from the same monomer groups with comparably good usage and processing properties, whose manufacture was to be connected with lower technological complexity than, for example, in the case of the aforementioned comb polymers.
A certain qualitative approximation of the above-described comb polymers could have been expected, perhaps, the soonest from the graft polymerization method, when the studies of the present Applicants had shown that the whole picture of the comb polymer properties is relatively insensitive to polymethylmethacrylate that is not bonded to the comb polymers. The road via graft polymerization to thermoplastically processible elastomers based on acrylates seemed closed, insofar as it turned out that, even under ideal graft conditions (feed polymerization, absence of transfer-active solvents and auxiliaries), no graft polymerizates are obtained that, with the same composition, achieve even only remotely the properties of the cited butyl acrylate-methyl methacrylate-macromonomer-comb polymers (e.g., tear strength at .delta.R=10 MPa; elongation at break .epsilon.R=400% and the like).
The polymers obtained thus were stickier, exhibited only low strength, and a pronounced tendency for stress whitening at low stress. Just as unsuccessful was an attempt to obtain elastomeric materials with somewhat satisfactory properties by grafting methyl methacrylate on preformed polyethylacrylate. All of these negative experiences with graft polymerization seemed largely preprogrammed in light of the experiences with block polymers reduced to the common denominator in the Encyclopedia of Polymer Science & Technology, vol. 5, loc. cit., page 417: "Most polymers are thermodynamically incompatible with other polymers and mixtures separate into two phases. This is true even when the polymeric species are part of the same molecule, as in these block copolymers."
In light of these failures with the representatives of monomers or polymers that were tested and generally rated as typical of their category by experts it seemed--as aforementioned--somewhat hopeless to arrive by means of a simple grafting of (meth)acrylate monomers on poly(meth)acrylates at elastomers, which meet the requirements of industry by achieving, e.g., the standard which was specified with the comb polymers known from European Published Patent Application 0 381 065.